The present invention has for its object an installation for storage of a liquified gas under pressure in a pressure chamber and a process for storage of a liquified gas under pressure in such a chamber.
Industrial gases such as oxygen, nitrogen, argon which are very widely used in all sectors of industry are distributed in large part in the form of liquid at low temperature and are stored at the user site in a cryogenic chamber most often called an xe2x80x9cevaporatorxe2x80x9d whose design pressure is generally comprised between 10 and 20 bars. The cost of the evaporators greatly increases the cost of industrial gases for the user. However, nearly half this cost comes from the metal, most often austenitic stainless steel, of which the pressure container is made that contains the cryogenic liquid. There is thus a real interest in being able to build chambers or evaporators whose cost would be reduced relative to the present cost.
Moreover, it will be understood that, for obvious safety reasons, the computation of the dimensions of the chamber or the vat must be carried out very carefully according to the standard EN 10 028-7 annex F.
In the accompanying FIG. 1, there is shown such an installation of known type, which is constituted by the cryogenic chamber 12 resistant to pressure, in which liquified gas 14 is stored. The installation also comprises a withdrawal conduit 16 at the lower part of the vat as well as a safety valve 18 connected to the upper portion of the chamber 12. Most often, the installation is also provided with a system 20 for detection of the level of the liquid 22 in the vat. This system 20 permits giving the percentage of the height of the chamber occupied by the liquid and is based on a differential pressure measurement. This measurement permits controlling the filling of the chamber when the percentage falls below 30%.
In computations aimed at determining the dimensional characteristics of the cryogenic chamber which use the standard mentioned above, there is taken into consideration a temperature equal to the ambient temperature as well as a service pressure which corresponds to the opening pressure of the safety valve 18.
A first object of the invention is to provide a storage installation for a liquified gas under pressure, which permits lowering the cost substantially in that it relates to the quantity of metal used for the pressure chamber whilst maintaining of course the safety conditions strictly equivalent to those which are required by the standards.
To achieve this object according to the invention, the storage installation for a liquified gas under pressure in a pressure chamber comprises:
a closed chamber resistive to pressure to contain said liquid, said chamber comprising a wall whose thickness is determined by computations taking into account the parameters connected with a pressure Ps within said chamber and at a temperature Ts less than xe2x88x9250xc2x0 C. of the wall,
means to work out a magnitude G representative of the effective temperature Te of the wall of the chamber, means to work out a magnitude Gxe2x80x2 of the same nature as G and representative of the computation temperature Ts,
means to compare the magnitudes G and Gxe2x80x2,
means to lower the pressure to a value P2 less than the calculation pressure Ps if it results from the comparison that Te is greater than Ts.
It will be understood that the invention is based on the one hand on the fact that the inventors have shown that so long as the quantity of liquid of the liquified gas contained in the vat is at least equal to 10%, the temperature of the wall of the chamber remains very much lower than the ambient temperature taken generally as a parameter for computation and that, more precisely, this temperature remains less than at least xe2x88x9250xc2x0 C. and more precisely than xe2x88x9280xc2x0 C. As a result, the computations of determination, particularly the thickness of the wall of the pressure chamber, are made on the basis of this temperature, which permits finding very substantially reduced thicknesses and hence a decrease of the quantity of steel to be used. On the other hand, the installation is so constructed that if the effective temperature of the chamber wall exceeds the temperature considered by the calculations, the valve is automatically controlled to drop the pressure within the chamber to a pressure substantially lower than that which was used for computing the thickness of the wall, whereby this temperature increase is compensated by the pressure decrease as to the requirements to which the chamber wall is subject.
According to a first embodiment, the magnitude G is the temperature itself in the chamber of the pressure receptacle.
According to a second embodiment, the magnitude used is the percentage of the height of the liquid contained in the pressure resistant chamber, which height is directly as was established by the inventors relative to the external temperature of the chamber wall. This modified embodiment has the advantage of using a liquid level detector in the pressure resistant chamber, which already exists in most installations.
Another object of the invention is to provide a process for the storage of liquified gas under pressure in a chamber.
This process is characterized in that it comprises the following steps:
the thickness of the wall of said chamber is calculated using parameters corresponding to a pressure Ps within the chamber and a temperature Ts (Ts less than xe2x88x9250xc2x0 C.) of said chamber, from which is obtained a thickness es,
a pressure resistant chamber is made whose thickness is equal to es,
said chamber is filled with said liquified gas,
a magnitude G is measured, representative of the effective temperature Te of the wall of said chamber as said gas within the chamber is progressively withdrawn,
said measured magnitude G is compared to a reference magnitude (Gxe2x80x2) of the same nature as the measured magnitude G, representative of said temperature Ts,
pressure in the chamber is lowered if the temperature Te becomes greater than the temperature Ts.